Metallizing process for glass-free ceramics bodies

ABSTRACT

This invention relates to sintered, substantially glass-free, ceramic bodies, and more particularly to an improved method of producing a metallized surface on such ceramic bodies. The method includes coating a sintered ceramic body with a metallizing ink composition containing a metal from the group molybdenum, tungsten, and mixtures of each of such metals with manganese, and subsequently firing the coated ceramic body in a wet reducing atmosphere to oxidize the metal from said first-mentioned group to yield a metallic layer which is tightly bonded to the surface of the ceramic body. The oxide of the metal from said firstmentioned group bonds directly to the ceramic body which in turn mechanically adheres the metal layer, from said second-mentioned group, to the ceramic body.

United States Patent s ,as as Schwyn et a1. Jan. 25, 1972 [5 METALLIZINGPROCESS FOR GLASS- OTHER PUBLICATIONS FREE CERAMICS BODIES R. D. Bagleyet al., Effect of T10, on Initial Sintcring of A1, O An Article A earinin the March 1970 issu of the 72 In to.Ra dE.Sh ,Fl t; B, PP g e ven rszgz g g i? Mon-ls. erg Journal of the American Ceramic Society at pages136- 141. Also Presented at the 65th Annual Meeting of the ACS, Pitt-[73] Assignee: General Motors Corporation, Detroit, sburg, Pa. Apr. 30,1963 (Basic Science Division No. 9- B- Mich. 63) E. J. Smoke et al.,Hydrogen & Vacuum Firing" An Article [22] Flled' July 1969 Appearing inthe Apr., 1965 issue of Ceramic Age at pages [21] Appl. No.: 840,007103- 104.

Related US. Application Data [63] Cgginubatiojn-in-gian of Ser. No.597,868, Nov. 30, Primary Examiner nonald JAmold a an one AssistantExaminer-John H. Miller 1 d t .T h 52 us. or ..117 227, 117/46 CA,117/160, 'i r r Carter r? 9 T?" 264/61, 264/62, 264/65 [51] 1nt.Cl...C04b 41/14 58 Field at Search ..264/61, 65, 62; 117/22, 62.9, 1571ABSTRACT 117/1195 160; 106/]; 252/515 518 This invention relates tosintered, substantially glass-free, ceramic bodies, and moreparticularly to an improved method [5 6] References Clted of producing ametallized surface on such ceramic bodies. The

method includes coating a sintered ceramic body with a metal- UNITEDSTATES PATENTS lizing ink composition containing a metal from the group2,613,153 10/1952 Stafford ..106/41 molybdenum, tungsten, and mixturesof each of such metals 3,473,987 10/1969 Soward ,264/59 with manganese,and subsequently firing the coated ceramic 3,3 1 2,533 4/1967 Botden etal... ...264/60 body in a wet reducing atmosphere to oxidize the metalfrom 3,403,043 9/1968 Thompson..... .117/160 said first-mentioned groupto yield a metallic layer which is 2,667,427 1/1954 Nolte ...117/22tightly bonded to the surface of the ceramic body. The oxide 2,667,4321/1954 None. 117/22 of the metal from said first-mentioned group bondsdirectly to 2,835,967 5/1958 Umblia ..117/22 h ami o y whi h in rnmechanically adhere the 3,296,017 1/1967 Rubin ..1 17/169 metal l y r.fr m i second-mentioned g p, to the r m- 3,446,643 5/1969 Karlak ..106/1ic body- FOREIGN PATENTS OR APPLICATIONS 639,124 3/1962 Canada ..1 17/227 5 Claims, 1 Drawing Figure METALLIZING PROCESS FOR GLASS-FREE CEBODIES This application is a continuation-in-part of copendingapplication Ser. No. 597,868, filed Nov. 30, 1966, now abandoned.

Hermetic ceramic-to-metal seals having high strength are required in themanufacture of high-vacuum tube envelopes and other feedthroughinsulators for electrical devices. In making these ceramic-to-metalseals it is necessary to form a vacuumtight bond between the ceramicbody and the metal. This is generally accomplished by metallizing thesurface of the ceramic body; that is, bonding a thin metallic film ontothe surface of the ceramic body and then bonding this metallized surfaceto the metal portion of the electrical device by conventional means suchas brazing. Most commonly used metallizing processes involve the use ofmolybdenum and manganese metals in which the manganese reacts with thewater vapor when heated in a wet, hydrogen atmosphere to form manganeseoxide. The manganese oxide reacts with the glass phase in the ceramic toform a compound which melts to form a bonding layer. This bonding layeradheres tightly to both the surface of the glass-containing ceramic bodyand to the thin metallic film of molybdenum. in metallizing ceramicbodies which are relatively glass-free, it has in the past beennecessary to add silica, calcia, kaolin, or other glass-formingmaterials to the metallizing inks in order to promote the formation ofthe bonding layer.

Metallizing methods which depend upon a glass phase to bond the metalfilm to the surface of the ceramic body such as those described above,have several disadvantages. The metal film produced by these methods hasa glass phase therein as a result of absorbing during the sintering stepsome of the glass from the bonding layer. The presence of the glassphase in the metal film tends to cause the metal film to havelow-thermal conductivity and low-electrical conductivity. For example,if too much glass is present in the metal film, the result is a glassysurface which has a relatively high-electrical resistance and which isdifficult to braze metals thereto. Another disadvantage with themetallizing methods described above is that the glass phase in thebonding layer and in the metallic film layer may react with constituentsof the environment in which some of the electrical devices are operatedthereby causing a failure. For example, metallized alumina bodiescontaining silica were found to react when exposed to cesium. Sincethese conventional fired ceramic metallizing methods require a glassphase either as a constituent of the ceramic body or as a material addedto the ink, they are not suitable for forming glass-free metallizedceramic bodies.

it is a primary object of this invention to provide an improved methodof metallizing substantially glass-free sintered ceramic bodies. It isanother object of this invention to provide an improved method ofmetallizing high-purity sintered alumina bodies. it is yet anotherobject of this invention to form a substantially glass-free metallizedceramic body in which the metallic coating has high-thermal conductivityand high-electrical conductivity.

These and other objects are accomplished by metallizing a sinteredsubstantially glass-free ceramic body with a metallizing ink compositioncontaining fine grained powder taken from the group consisting ofaluminum, magnesium and beryllium. A specific and preferred embodimentin accordance with the practice of this invention is a metallizing inkcomposition containing a liquid vehicle, aluminum powder and arefractory metal such as molybdenum powder. The mixture may contain asmall amount of nickel powder to aid in sintering. The metallizing inkcomposition is sprayed or otherwise suitably applied onto the surface ofa high-purity sintered alumina body which is then fired in a wetreducing atmosphere to yield a metallic layer tightly bonded to thesurface of the highpurity alumina body. The aluminum powder in themetallizing ink composition is oxidized and is sintered tightly to thealumina body and enables the sintered molybdenum to adhere or bondtightly to the alumina surface. The metal film on the surface of thehigh-purity sintered alumina body is essentially glass-free and, as aresult, has relatively high-thermal conductivity, high-electricalconductivity, and is chemically inert to materials reactive with glass.We have found that the alumina formed in the powder mixture duringfiring and while in intimate contact with the ceramic body sintersreadily to the body forming the desired tight bond even in the absenceof glass. We have found that the aluminum does not serve to fuse therefractory metal powder into a solid compact but instead forms a bond tothe ceramic body which adheres the refractory metal layer mechanicallyto the ceramic. The firing temperature is high enough to sinter therefractory metal powder into a continuous film with alumina dispersedthroughout.

In general, the process of our invention may be carried out as follows.A powder taken from the group consisting of aluminum, beryllium, andmagnesium is mixed with one or more of the well-known refractory metalssuch as molybdenum and tungsten to form a powdered metallizing mixture.The concentration of the aluminum, beryllium, and magnesium in thepowdered metallizing mixture is from about 15 to 40 weight percent,aluminum being the preferred metal powder of this group and is preferredin an amount of about 20 to 30 percent. The particle size of thealuminum powder is less than 50 microns with the preferred size beingless than 40 microns. We have found it necessary that there be nocoating on the powder grains to insure oxidation. The particles size ofthe molybdenum and tungsten refractory metallizing metals used in themixture is less than 5 microns with the preferred particle size of thesepowders being 2 to 5 microns. The use of fine sized powder is necessaryto enable intimate bonding in the manner described above. Whereas coarsegrained powders can be used in making compacts, such cannot be used inour process. The concentration of these well-known refractory metals,for example, molybdenum, in this mixture is from about 60 to 85 weightpercent. A mixture of molybdenum and manganese as well as a mixture oftungsten and manganese in which the mixtures contain about 5 to 40weight percent manganese may be used in place of the molybdenum ortungsten. The concentration of the nickel powder, when used, is 0.01 to1.0 weight percent. We have found that poor bond strength is obtainedwhere the amount of aluminum or other such powder is less than about 15weight percent. Where the amount exceeds about 40 percent we find itdifficult to braze to due to poor wetting characteristics.

As is a common practice in the art, the metallizing powder mixture ismixed with a liquid vehicle to form a metallizing ink composition whichis applied onto the surface of the body by brushing, spraying,screening, or other suitable methods. Any suitable liquid vehicle whichis commonly used in metallizing ink compositions can be employed in thepractice of this invention such as any of the well-known squeegee oilsused in silk screen printing, or a mono or dialkyl ether, for example, adibutyl ether, of diethylene glycol solution containing 2 to 4 weightpercent ethyl cellulose. The concentration of the liquid vehicle in themetallizing ink composition is in the amount of about 25 weight percentof the powder mixture. The liquid vehicle is completely bumed offwithout residue during firing and is important only as a means ofretaining the metal powder on the ceramic body during processing. It ispreferred that bum-off take place at temperatures of less than about 600F.

The metallizing ink composition of this invention has been successfullyapplied to alumina, beryllia, and zirconia ceramic bodies, bodiesnormally difficult to metallize in the absence of glass. For example,substantially glass-free, high-purity alumina bodies having a content of99 percent or more A1 0 have been successfully metallized in accordancewith this invention.

In accordance with the teachings of our invention, after the ceramicbody has been coated with a film or layer of the metallizing ink, it isthen heated in a moist reducing atmosphere such as disassociatedammonia. An atmosphere having a dew point of about F. is convenientlyobtained by passing the gas mixture through water at room temperature.Other reducing atmospheres known in the art, i.e., wet forming gas, arealso satisfactory. The degree of moisture is not critical and need beonly that sufiicient to cause oxidation of the aluminum or like metal.The coated ceramic body is fired in this reducing atmosphere for aperiod of approximately 30 minutes at a temperature ranging from 2,200to 3,200 F. depending on the firing temperature required to sinter therefractory metal mixture. Firing under these conditions sinters themetal mixture to form a metallic layer which adheres or bonds tightly tothe ceramic body.

Typical examples in accordance with the practice of this invention formetallizing sintered ceramics are as follows.

EXAMPLE NO. 1

A high-purity alumina body containing 99 weight percent alumina, 0.12weight percent MgO and 0.10 weight percent Si was coated with ametallizing ink composition which contained about 26 parts by weightaluminum, particle size less than 38 microns; about 74 parts by weightmolybdenum powder, particle size 2 to 5 microns; approximately 0.01parts by weight nickel powder; and approximately 25 parts by weight of acommercially available squeegee oil such as is used in conventional silkscreen printing. The coated alumina ceramic body was fired at 3,000 F.for 30 minutes in a disassociated ammonia atmosphere having a dew pointof approximately 80 F., producing an outer metallic layer which wastightly bonded to the ceramic body.

EXAMPLES 2 THROUGH 8 The following table lists examples 2 through 8 inwhich different ceramics were successfully metallized by the methodstaught in accordance with our invention.

' 30 minutes in a disassociated ammonia atmosphere having a dew point of80 F.

The data in the table shown above indicates that the metallizing inkcomposition can be used on high-purity alumina, beryllia and zirconiabodies, there being glass-free bodies, as well as steatite andforsterite bodies which do contain glass and thus may also be metallizedby methods known in the art. The accompanying drawing diagrammaticallyshows a body fonned in accordance with our invention.

While the embodiment of the present invention herein disclosedconstitutes a preferred form, it is to be understood that other formsmight be adopted.

What is claimed is:

1. A method of metallizing the surface of a substantially glass-freealumina body comprising the steps of applying a metal powder coatingonto said surface; said coating consisting essentially of about 15 to 40weight percent of a fine metal powder taken from a first groupconsisting of aluminum and magnesium, about to 85 weight percent of afine metal powder taken from a second group consisting of molybdenum,molybdenum-manganese mixture, tungsten and tungsten manganese mixtureswhere said mixtures contain about 5 to 40 weight percent manganese, saidpowders being carried by a liquid vehicle which is completely burned offwithout residue at elevated temperatures, oxidizing the metal powderfrom said first group by heating said coated body at a temperature ofabout 3,000 F. in a moist-reducin atmosphere whereby the oxide of saidmetal powder taken rom said first group IS sintered to said body and isinterspersed in the metal taken from said second group to form a firmlyadherent coating on said sintered ceramic body having properties of highthermal and electrical conductivity, the metal powder taken from saidfirst group being less than 50 microns in size and the metal powdertaken from said second group being less than 5 microns in size.

2. A method as described in claim 1 wherein said metal powder in saidfirst group is aluminum.

3. A method as described in claim 2 wherein said second group metalpowder is molybdenum.

4. A method as described in claim 1 wherein said first group metal isaluminum in an amount of about 20 to 30 percent by weight and the metalin said second group is molybdenum in an amount of about -80 percent byweight.

5. A method as described in claim 3 wherein said first group metal isaluminum in an amount of about 26 percent by weight and the metal insaid second group is molybdenum in an amount of about 74 percent byweight and said coated body is held at firing temperature for about 30minutes.

Po-ww UNITED STATES PATENT OFFlCE CERTIFICATE OF CORREC'HQN Patent No. 3'637v435 Dated Jan. 0 1972 Inventor) Raymond E. Schwyn and Morris BergIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

r- In the title of the patent, line 2, "CERAMICS" should .1

read CERAMIC In the ABSTRACT, line 5, after "group" insert aluminum,magnesium, and beryllium and a metal from the group In the specificationtitle, C010 1, line 1, "CERAMICS" should read CERAMIC line 10,"vacuumtight" should read vaduum-tight line 48, after "the" insertmetallizing C01,, 2, line 26,, "particles" should read particle In theclaims, Col. 4, line 3, "there" should read these Signed and sealed this15th day of August 1972.

(SEAL) Attest:

EDWARD M.FLETOHER,JR. ROBERT GOTISCIH'ALK- Attesting OfficerCommissioner of Patents

2. A method as described in claim 1 wherein said metal powder in saidfirst group is aluminum.
 3. A method as described in claim 2 whereinsaid second group metal powder is molybdenum.
 4. A method as describedin claim 1 wherein said first group metal is aluminum in an amount ofabout 20 to 30 percent by weight and the metal in said second group ismolybdenum in an amount of about 70- 80 percent by weight.
 5. A methodas described in claim 3 wherein said first group metal is aluminum in anamount of about 26 percent by weight and the metal in said second groupis molybdenum in an amount of about 74 percent by weight and said coatedbody is held at firing temperature for about 30 minutes.